What Is Coral? A Coral Polyp and Zooxanthellae | Smithsonian Ocean
PDF | On Jan 1, , S. Karako and others published The Taxonomy and Evolution of the Zooxanthellae-Coral Symbiosis. Coral polyps, which are animals, and zooxanthellae, the plant cells that live within them, have a mutualistic relationship. Coral polyps produce carbon dioxide . Tiny plant-like organisms called zooxanthellae live in the tissues of many animals , including some corals, anemones, and jellyfish, sponges, flatworms, mollusks.
Their pink color comes from the zooxanthellae living inside. More about coral reef ecosystems can be found in our Coral Reefs featured story.
Bleached Corals, Pacific Ocean Credit: Wolcott Henry When the reef is under stress from high temperatures, pollution, or other threats, the zooxanthellae abandon their coral hosts in a process called "bleaching. When they die, just the white skeleton is left behind as if it had been bleached.
Bleaching spells trouble for coral reefs. The reef suffered a mass bleaching event in the summer ofwhen water temperatures were unusually high. Bleached corals have lowered defenses against disease, so often will suffer further damage and death as disease moves in after a bleaching event.
In this photo, healthy brown coral gives way to the frontlines of disease. Amanda Feuerstein It can be hard for coral to recover for a bleaching event. The best bet is the some healthy tissue still remains deep in the skeleton and, if conditions improve, this coral can grow and recover, spreading to the rest of the skeleton.
The discovery and research into these compounds also supported that the molecules were from the algae and not a result of the host, but it seemed that variation to the host and environment caused the production of different algal metabolites.
Many other toxins and compounds were isolated in this study and added significantly to the fact that the metabolism and taxon of zooxanthellae are extremely diverse.
Furthermore, it has been shown that specific Symbiodinium are more tolerant to heat and stress, and perhaps corals adopting these specific algae will be able to survive the temperature changes from global warming and natural disasters Another study found that following bleaching, corals had clade shuffled from C2 to D, because D has a higher densities and photochemical efficiency, resulting in higher thermal tolerance The coral polyps do cellular respiration, thus producing carbon dioxide and water as byproducts.
The zooxanthellae then take up these byproducts to carry out photosynthesis. The products of photosynthesis include sugars, lipids, and oxygen, which the coral polyps thus uptake for growth and cellular respiration, and the cycle continues.
The photosynthesis byproducts are more specifically used to make proteins and carbohydrates in order to produce calcium carbonate for the coral to grow. Furthermore, the oxygen is used by the coral to help remove wastes.
This recycling of nutrients in between these symbionts is extremely efficient, resulting in the ability to live in nutrient poor waters. About ninety percent of the material produced by photosynthesis is thought to be used by the coral 6.
Zooxanthellae and Coral Bleaching | Smithsonian Ocean
In terms of disease, the zooxanthellae is commonly the point of attack, rather than the coral itself. For example, the Montastrae species, which causes Yellow Band Disease, affects the zooxanthellae directly rather than the coral 7. Scientists found that a coral, Acropora, lacked an enzyme needed for cysteine biosynthesis.
It thus needed Symbiodinium for the production of this amino acid. The genome size for the zooxanthellae algae is about 1, Mbp while the coral is approximately Mbp: Sure enough, other studies have shown phosphate-linked relationships between these two species. Zooxanthellae extracted from the Acropora coral had two acid phosphatases P-1 and P The activity of these enzymes shows that perhaps their role is involved in the mobilization of a phosphate storage compound.
The exact role of these enzymes is unknown, but it seems that the symbiotic relationship between coral and zooxanthellae is phosphate limited But together, the coral and zooxanthellae can synthesize twenty amino acids 17 Figure 6. There is also a relationship between the amount of time the tentacles of the coral spend expanded or contracted and the amount of zooxanthellae present on the coral.
In general, there was lower photosynthetic efficiency in the zooxanthellae coral species that has their tentacles expanded only at night than the species with their tentacles constantly expanded. Also, the zooxanthellae density was higher in the continuously expanded tentacle species. These differences were found only in the light however, because when the species were placed in the dark no differences were found.
Thus the light has a relationship with the coral and zooxanthellae, which was assumed because zooxanthellae are photosynthetic organisms. Conclusively, the species with continuously expanded tentacles have dense populations or small tentacles. The findings suggest that small tentacles do not shade the zooxanthellae, thus they are all visible to the light, and that dense populations are necessary to harvest the light.
So the species with these proactive properties expand continuously to collect all the light, while the species with few zooxanthellae only expand at night Another study related the exposure of the coral to oxygen as a means for oxygen radical accumulation in its tissues The O2 concentrations were found to increase by a pH of about 1.
Thus causes an increase of oxygen radicals in the coral tissues from the molecular oxygen, and the radicals can destroy cells. This study found that the anemones with higher chlorophyll, and thus higher Symbiodinium, actually adjusted their protein expression so the fluctuating oxygen concentrations would not be destructive. This is just another example of how the coral changes its innate reactions to adjust for its symbiotic algae Figure 7.
Movement Furthermore, it was found that the temperate symbiotic sea anemone, Anthropluera balli, incorporates a maternal inheritance of the zooxanthellae because the anemone live in locations of low zooxanthellae algae. It was found that the spawned ova consistently contained zooxanthellae, and were released into the ocean water to become fertilized and grow.
The zooxanthellae was clearly integrated into the life cycle of this particular sea anemone, and was found to localize at one end of the embryo to become integrated within the endoderm, which as mentioned above is where the zooxanthellae live within coral This study brings arise the question of how zooxanthellae disperse among the coral. Another study discovered that the zooxanthellae can be released by the host in ways such as predation, extrusion, spontaneously, osmotically, or as we know, due to temperature or stress.
This particular study proposes another way for zooxanthellae to disperse, through the feces of their predators. Interestingly, photosynthetic rates from the unharmed species were very similar to the rates from the fecal zooxanthellae that made their way through a digestive tract. Furthermore, the zooxanthellae reinfected sea anemones after their travel through the digestive tract of their predator.
The engine of the reef: photobiology of the coral–algal symbiosis
This finding showed that predation is an important means by which the zooxanthellae are dispersed among a coral reef History The relationship between Symbiodinium and coral has been known for about fifty years.
One of the first studies found that certain dinoflagellates fixed labeled carbon from CO2 and moved it to their host sea anemone after forty-eight hours. This study also showed that Symbiodinium produced higher amounts of carbohydrates when living inside a host rather than free living After this symbiotic relationship was discovered, other studies delved further into how the algae and coral used the nutrients they acquired from the other.
One study found specifically that the algae fixed the carbon primarily as glycerol, which was then taken up by the coral tissue as proteins and lipids It was also discovered that the other organic acids produced by the Symbiodinium were different biochemically, even though they looked the same This information was the beginning of other scientists discovering the increasingly wide variety in the taxon of dinoflagellates.
It is not entirely sure how the coral does this, but some studies have hypothesized. Other studies suggest that the host coral produces compounds that act as host release factors, and that these factors can control the metabolite production in the Symbiodinium Energy Storage Not only are nutrients shared between the two species, but energy and energy production is integrated as well. The Symbiodinium produced these lipids, using acetate from the coral and extra ATP, and excreted them back to their host.
These lipids are mostly wax esters and triglycerides A figure showing the decline in zooxanthellae over a starvation period http: It was further shown that the retention of this ammonium by the coral was related to the Symbiodinium because the algae uptakes most of the ammonium itself The algae were also more efficient with its use of a nitrogen source because it can use nitrite.
A study used tagged enzymes involved in the use of different forms of nitrogen, and concluded that the algae do indeed utilize nitrates.
They also found that the algae densities increase with the nitrate concentration, although further details of this relationship with the coral are not known It is also interesting to note that the MAA concentration, which usually increases with UV exposure, also increased at high ammonium concentrations This study was done in red algae, Porphyra, but still may provide information regarding the zooxanthellae and its symbiotic relationship with corals Figure 8.
Human Threat Figure 9. Some fishing practices involve blowing up reefs with explosives to stun the fish so the fisherman can catch them easily Figure 9. Another fishing practice that is particularly detrimental is fishing with cyanide. Divers pour cyanide, a poison, on the reefs to stun the fish. The divers also directly rip coral off the reef to catch the hiding and sick fish.
These practices of fishing are completely destroying the reefs and environment. Also, as we saw above, some fish that are predators of the zooxanthellae actually disperse the algae in their feces. Due to overfishing, this dispersion technique may no longer be available, thus diminishing the diversity of zooxanthellae, and therefore coral, around the oceans.
- Zooxanthellae and their Symbiotic Relationship with Marine Corals
Also, coral is very delicate, and divers merely touching the coral can damage years of growth. It is also thought that the oils from a human can be harmful towards the coral and algae living within or on it; tourism perhaps has been degrading coral for years.